The invention relates generally to a scanner for producing an image of an original, e.g., a photographic negative or diapositive.
More particularly, the invention relates to an electronic scanner having a sensor with a single row of sensing elements which function to sense the light from an illuminated original. The original is scanned by moving the sensor and the original relative to one another.
The general arrangement of prior art scanners is shown in the exploded view of FIG. 1 where the reference numeral 2 identifies an original to be scanned. The original 2 is illuminated by a light beam 1 and the light which passes through the original 2 is sent to a sensor 4 having a single row of sensing elements. An objective 3 serves to focus the light on the sensor 4. The original 2 is flat and, in order to scan the latter, the sensor 4 is moved in a direction perpendicular to the row of sensing elements as indicated by the double-headed arrow 5. The sensing elements then traverse the original 2 so as to scan successive increments of the original 2 at a multiplicity of points defining a series of rows and columns. Instead of moving the sensor 4 relative to the original 2, the original 2 can be moved relative to the sensor 4 so that the increment of the original 2 focused on the sensor 4 always passes through the optic axis.
As illustrated in FIG. 2, a protective sheet 7 of glass overlies the sensor 4 in selected models of prior art scanners. If a video signal of high quality is to be obtained during scanning, the characteristics of the sensor 4 must be such that the contrast in the video signal is as great as that in the original 2, e.g., three density gradations which is equivalent to a noise-to-signal ratio of 1:1000 in the video signal. Scattered light in the light path plays a very significant role here. FIG. 2 shows a group of light rays 6 which impinge upon the protective sheet 7 of the sensor 4 at the location A of FIG. 1. The protective sheet 7 produces scattered light, indicated by the arrows 8, 9, 10 and 11, as the rays 6 enter and leave the protective sheet 7. The scattered light impinges upon the sensor 4 at locations which are not illuminated by the rays 6 and thus reduces the contrast in the video signal. Furthermore, since the surface of the sensor 4 is reflective as a result of the chip manufacturing technique used for the sensor 4, the sensor 4 reflects light to the protective sheet 7 as shown, for example, by the arrows 12 and 13. The protective sheet 7, in turn, reflects this light back to the sensor 4 as indicated by the arrows 14 and 15 thereby generating additional scattered light.
At a boundary of high contrast between light and dark regions, the scattered light introduces a base signal of predetermined magnitude into the video signal of the line being scanned and also produces a halo about the edges of the boundary. Accordingly, it is not possible to achieve an abrupt change in signal such as, for instance, at a boundary of high contrast between light and dark regions.
FIG. 3 is a graphical representation of the video signal obtained with an arrangement of the type illustrated in FIG. 2. The position of the scanned column is plotted on the abscissa of FIG. 3 while the logarithm of the output voltage of the sensor 4 is plotted on the ordinate. The range of the ordinate is from log output voltage=0 to log output voltage=3 which corresponds to a variation of three decimal places in the output voltage. The portion of the output voltage responsible for the halo at the light-dark boundary is clearly seen at 16. The base signal which is due to the scattered light impinging upon the sensor 4 and contains electronic noise signals is indicated at 17. The ideal video signal is shown by broken lines 18.